Image of MACSJ0717 (1 of 2) (image)

Caption

A labeled version of the MACSJ0717 image shows the galaxies in the four different clusters -- identified by the letters "A," "B," "C," and "D" -- involved in the collision, plus the direction of motion for the three fastest moving clusters. The length of the arrow shows the approximate speed in a direction perpendicular to the line of sight. Note that the direction of motion of the clusters is roughly parallel to the direction of the filament. Data from Keck Observatory was used to derive the speed of the clusters along the line of sight, allowing the three-dimensional geometry and dynamics of MACSJ0717 to be derived.
The cooler (redder) region of gas towards the lower left of the cluster labeled "D" has likely survived from before the collision. Cluster A is likely falling back into the main cluster after already having passed through once in the opposite direction. Both of these clusters probably originated from the filament. Cluster B, however, has a much higher speed than the other clusters along the line of sight and its origin is unclear. It may have fallen along the outer edge of the filament, causing its infall trajectory to curve, or it may be falling in along another, smaller filament. The good alignment between the galaxies and hot gas for cluster C. along with its motion compared to MACSJ0717 as a whole makes this system a good candidate for the core of the main cluster.
The large region of relatively hot gas (shown in blue) that extends from the left side of cluster C to the right side of region D may be caused by heating as significant quantities of gas from the filament plough into the main cluster.

Credit

X-ray: NASA/CXC/IfA/C. Ma et al.
Optical: NASA/STScI/IfA/C. Ma et al.

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